Tracing control system

ABSTRACT

A tracing control system for machining a workpiece through tracing by calculating speed command values of respective axes, using change amounts detected by a tracer head, and moving a cutter relative to the workpiece through control of the speed of the respective axes in accordance with the speed command values. The system is provided with calculating means for calculating predetermined values in proportion to amounts of change in the speed command values of the respective axes, and a U-axis motor and a W-axis motor for moving only a model in parallel with the above respective axes by amounts corresponding to the respective predetermined values. A sub-table, on which only the model is placed is provided on a main table of a tracing machine tool, is moved in the same direction as the direction of movement of the main table, by an amount proportional to the amounts of change in the speed command values, for example, the amounts of delay in the reactions of the servo systems. Thus, the tracer head always precedes the cutter. When the tracer head comes to a corner and the axis speed is reduced, the sub-table is returned by the amount corresponding to the reduction in the axis speed. This return amount corresponds to the amount of over-run of the tracer head in the axial direction. Because the amount of movement of the cutter is reduced by the over-run amount, overshot machining is prevented.

DESCRIPTION Background of the Invention

The present invention relates to a tracing control system, and moreparticularly, to a tracing control system by which overshot machining atcorners and the like of a workpiece is prevented.

A tracing control system uses a tracer head to detect amounts ofdisplacement of respective axes applied to a stylus, and speed commandvalues of the respective axes are calculated in a tracing calculatingcircuit by using these amounts of displacement. A cutter is movedrelative to a workpiece by drive motors of the respective axes, inaccordance with these speed command values, and at the same time, thetracer head is moved along the surface of a model at the same speed.This motion is repeated, and accordingly, the workpiece is machined tothe same shape as the model.

Tracing control systems of this kind, however, have a problem in thatregions of the model at which the shape is abruptly changed cannot beprecisely traced. Namely, when the stylus comes to corners and the likeof the model, an over-run of the tracer head occurs due to a delay inthe reaction of the servo systems, and as a result, the overshotmachining into the workpiece by the same amount of over-run, and thusthe machining precision at corners is lost.

SUMMARY OF THE INVENTION

The present invention has been created in view of the aforesaiddrawbacks, and an object thereof is to provide a tracing control systemby which overshot machining at corners, etc., of a workpiece isprevented, and thus the tracing control system has a high precision.

To achieve the above object, in accordance with the present inventionthere is provided a tracing control system for machining a workpiecethrough tracing by calculating speed command values of respective axes,using change amounts detected by a tracer head, by moving a cutterrelative to the workpiece through a control of the speed of therespective axes in accordance with the speed command values and bymoving the above tracer head along the surface of the model, comprisingcalculating means for calculating predetermined values in proportion tothe amounts of change in the above speed command values of therespective axes, and a movement control means for moving only the modelin parallel with the above respective axes by amounts corresponding tothe above respective predetermined values.

A sub-table on which only the model is placed is provided on a maintable of a tracing machine tool. This sub-table is moved in the samedirection as the direction of movement of the main table, by an amountproportional to the amount of change in the speed command value, forexample, the amount of delay in the reaction of a servo system. As aresult, the tracer head always precedes the cutter, and when the tracerhead comes to a corner and the axis speed is reduced, the sub-table isreturned by the amount corresponding to the reduction in the axis speed.This return amount corresponds to the amount of over-run of the tracerhead in the axial direction, and thus the amount by which the cutter ismoved is reduced by the amount of this over-run of the tracer head,whereby overshot machining is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the hardware of a tracing controlsystem according to an embodiment of the present invention;

FIG. 2 is a diagram showing the arrangement of a tracing machine toolconnected to a tracing control system according to an embodiment of thepresent invention;

FIG. 3 is an illustration showing a corner of a model; and

FIG. 4 is a graph showing movement of a tracer head and a cutter at acorner, in accordance with the embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will hereinafter be describedwith reference to the drawings.

FIG. 1 is a block diagram showing the hardware of a tracing controlsystem according to an embodiment of the present invention. In FIG. 1, aprocessor 11 reads a system program stored in a ROM 12, through a bus10, and controls the overall operation of a tracing control system 1according to the system program. A RAM 13 stores amounts εx, εy, εz ofdisplacement of respective axes detected by a tracer head, which will bedescribed later, and other temporary data. A nonvolatile memory 14 isbacked up by a battery (not shown), and stores various parameters suchas tracing directions, tracing speeds, etc., entered from a controlpanel 2 through an interface 15.

The processor 11 generates speed command values Vx and Vz of therespective axes based on the amounts εx, εy, εz of displacement of therespective axes, and the commanded tracing direction and tracing speed,according to the known process. The speed command values Vx and Vz arethen converted to analog values by D/A converters 16x and 16z, amplifiedby servo amplifiers 17x and 17z, and output. Accordingly, each of themain motors of the X-axis and the Y-axis of the tracing machine tool,which will be described later, is driven.

At the same time, the speed command value Vx and Vz are divided toobtain values 1/Gx and 1/Gz at calculating circuits 18u and 18w,respectively. (Note, Gx and Gz are gains of the servo systems of theX-axis and the Y-axis, respectively.) The divided speed command valuesare then converted into command values Su and Sw at converting circuits19u and 19w. The command values Su and Sw are converted to analog valuesby D/A converters 20u and 20w, amplified by servo amplifiers 21u and21w, and output. Accordingly, each of the sub-motors of the U-axis andthe W-axis of the tracing machine tool is driven.

FIG. 2 shows the arrangement of a tracing machine tool connected to theabove tracing control system. In FIG. 2, a tracer head 31 detectsamounts of displacement εx, εy, εz in each of the X-axis, the Y-axis andthe Z-axis directions, generated by contact of a stylus 32 at the tipend of the tracer head with a model 33, and inputs the detected amountsto the tracing control system 1.

The tracer head 31 and a cutter 34 are both attached to a column 35. Thecolumn 35 is moved in the Z-axis direction by a main motor 36z. Further,a main table 37 is moved in the X-axis direction by a main motor 36x.Furthermore, in addition to these main motors, a sub-motor 39u formoving only a sub-table 38, on which the model 33 is placed, in theU-axis direction in parallel with the X-axis, and a sub-motor 39w forsimilarly moving only the sub-table 38 in the W-axis direction inparallel with the Z-axis are provided in the main table 37. A workpiece40 is fixed on the main table 37.

The main motors 36x and 36z are driven in accordance with theabove-mentioned speed command values Vx and Vy, and the sub-motors 39uand 39w are driven in accordance with the command values Su and Sw.

Note, the command values Su and Sw are proportional to the valuesacquired by dividing the speed command values Vx and Vy by gains of theservo systems of the X-axis and the Y-axis, respectively, i.e., inproportion to delays in the reactions of the servo systems of therespective axes. Accordingly, when tracing is started, the tracer head31 is always moved ahead of the cutter 34 by this delay amount.

When the tracer head comes to, for example, a corner CN of the modelshown in FIG. 3, and the speed in the X-axis direction is reduced (0 inthis case) at this time, immediately thereafter the sub-table 38 startsto move in a direction opposite to the preceding direction. Thus, theamount of precedence of the tracer head in the X-axis direction iscanceled out. Namely, this time the cutter 34 is at the same position asthe tracer head 31, as shown in FIG. 4.

In FIG. 4, the axis of the ordinate shows the position and the axis ofthe abscissa shows time. A continuous line shows the movement of thetracer head, and a dotted line shows the movement of the cutter. Whenthe tracer head over-runs the model, the positions of the cutter and thetracer head match when the tracer head returns from the over-runposition to the normal position. Namely, the amount of movement of thecutter in the X-axis direction is reduced by the amount of over-run ofthe tracer head. Thus an unwanted excess machining does not occur.

Note, the movement between the column and the main table and between thetracer head and the sub-table are carried out along two axes in theabove explanation, but the number of axes can be appropriately increasedor decreased.

With the present invention, as described above, as predetermined valuesin proportion to the amounts of change in the speed command values areobtained and only the model is moved by these amounts, by positioncommand values, even if the tracer head over-runs the model at cornersand the like, the cutter does not bite into the workpiece. Thus theprecision of the tracing machining is improved.

We claim:
 1. A tracing control system for machining a workpiece throughtracing by calculating speed command values of respective axes, usingchange amounts detected by a tracer head, by moving a cutter relative tothe workpiece through a control of the speed of the respective axes inaccordance with the speed command values and by moving the tracer headalong the surface of a model, comprising:calculating means forcalculating predetermined values in proportion to the amounts of changein the speed command values of the respective axes; a main table; and asub-table provided on said main table for moving only the model inparallel with the respective axes, said sub-table driven by sub-motorsinstructed by amounts corresponding to respective ones of thepredetermined values.
 2. A tracing control system according to claim 1,wherein said respective predetermined values represent amounts of delayin reactions of servo systems of the respective axes.
 3. A tracingcontrol system according to claim 2, wherein said amounts of delay areobtained by dividing the speed command values by gains of the servosystems of the corresponding axes.